cvs commit: src/sys/kern vfs_subr.c

[Alfred Perlstein]

* Jeff Roberson <jroberson at chesapeake.net> [050615 21:47] wrote:
> On Thu, 16 Jun 2005, Jeff Roberson wrote:
> 
> > jeff        2005-06-16 04:41:42 UTC
> 
> This fixes duplicate frees from the vnode zone, crashes in vnlrureclaim
> and vlru_free() and a few other related problems.  Basically, I've
> converted the holdcnt to be a proper ref counting system, while use count
> maintains the traditional vfs use count semantics, which are not really
> compatbile with structure life-time issues.  It looks like a big change
> but it really makes things much simpler.  Briefly, the rules are this:
> 
> 1) vget/grab a use count if you're a normal consumer of the vnode API.
> That goes for syscalls, mmaping, nfs, etc.
> 
> 2) vhold() if you want to prevent a vnode from being recycled while you
> hold a reference to it.  The name cache uses this, as does the vm when
> pages reference the vnode's vm object.  This is also used for vlrureclaim,
> vflush, vtryrecyle, etc. which want to make sure the vnode is not recycled
> while it has a local reference.
> 
> I'll probably add a comment to vnode.h to describe this in more detail.

Please do, last I remember, vhold and vget are the same (gaining a
ref), except vget can return the vnode locked.  This is a bit confusing.

> 
> >
> >   FreeBSD src repository
> >
> >   Modified files:
> >     sys/kern             vfs_subr.c
> >   Log:
> >    - Change holdcnt use around vnode recycling.  We now always keep a holdcnt
> >      ref while we're calling vgone().  This prevents transient refs from
> >      re-adding us to the free list.  Previously, a vfree() triggered via
> >      vinvalbuf() getting rid of all of a vnode's pages could place a partially
> >      destructed vnode on the free list where vtryrecycle() could find it.  The
> >      first call to vtryrecycle would hang up on the vnode lock, but when it
> >      failed it would place a now dead vnode onto the free list, and another
> >      call to vtryrecycle() would free an already free vnode.  There were many
> >      complications of having a zero ref count while freeing which can now go
> >      away.
> >    - Change vdropl() to release the interlock before returning.  All callers
> >      now respect this, so vdropl() directly frees VI_DOOMED vnodes once the
> >      last ref is dropped.  This means that we'll never have VI_DOOMED vnodes
> >      on the free list.
> >    - Seperate v_incr_usecount() into v_incr_usecount(), v_decr_usecount() and
> >      v_decr_useonly().  The incr/decr split is so that incr usecount can
> >      return with the interlock still held while decr drops the interlock so
> >      it can call vdropl() which will potentially free the vnode.  The calling
> >      function can't drop the lock of an already free'd node.  v_decr_useonly()
> >      drops a usecount without droping the hold count.  This is done so the
> >      usecount reaches zero in vput() before we recycle, however the holdcount
> >      is still 1 which prevents any new references from placing the vnode
> >      back on the free list.
> >    - Fix vnlrureclaim() to vhold the vnode since it doesn't do a vget().  We
> >      wouldn't want vnlrureclaim() to bump the usecount since this has
> >      different semantics.  Also change vnlrureclaim() to do a NOWAIT on the
> >      vn_lock.  When this function runs we're usually in a desperate situation
> >      and we wouldn't want to wait for any specific vnode to be released.
> >    - Fix a bunch of misc comments to reflect the new behavior.
> >    - Add vhold() and vdrop() to vflush() for the same reasons that we do in
> >      vlrureclaim().  Previously we held no reference and a vnode could have
> >      been freed while we were waiting on the lock.
> >    - Get rid of vlruvp() and vfreehead().  Neither are used.  vlruvp() should
> >      really be rethought before it's reintroduced.
> >    - vgonel() always returns with the vnode locked now and never puts the
> >      vnode back on a free list.  The vnode will be freed as soon as the last
> >      reference is released.
> >
> >   Sponsored by:   Isilon Systems, Inc.
> >   Debugging help from:    Kris Kennaway, Peter Holm
> >   Approved by:    re (blanket vfs)
> >
> >   Revision  Changes    Path
> >   1.632     +198 -202  src/sys/kern/vfs_subr.c
> >

-- 
- Alfred Perlstein
- email: bright at mu.org cell: 408-480-4684